To have commitments the next day, esp. in the morning, eg interviews, work, dates etc, that prevents you from doing something (usually fun and stupid) on that night.

Originated, (obviously) when having to go to bed early because school was the next day.

When you eat dirt for the first time and leave your family to build a shack in the woods

Left over from when you'd stay up too late when you had school the next day, but applied to adult life; For example going clubbing when you have work the next day

An excuse to prevent from doing an immediate action or otherwise on a later date.